1. Field of the Invention
The present invention relates generally to light emitting devices in the form of a single semiconductor chip and capable of emitting light of white color, neutral colors between red and blue, i.e., magenta or pink and the like, and particularly to structures improving a spatial distribution of a color of light emitted thereby.
2. Description of the Background Art
As a material for a high bright light emitting diode (LED) a device including a light emitting layer of AlGaAs, GaAsP and the like for red color has been put to practical use and an intensity of no less than several candela (Cd) has been implemented by an inexpensive LED. Other than LEDs for red, GaP for green and yellow-green, GaInN for blue and green, AlGaInP for Mars yellow and yellow have all been put to practical use as inexpensive LEDs.
These LEDs, however, all use a single semiconductor material for their light emitting layers and in principle they can only provide monochromatic light emission. Thus, while typical LEDs are capable of emitting light of a primary color, a color between red and green, and a color between green and blue, such as red, Mars yellow, yellow, yellow-green, green, blue-green, blue, blue-violet and violet, they are incapable of emitting light of a neutral color between red and blue and a neutral color between red, green and blue.
For illumination, decoration and some other similar applications for displaying, a source of light of a color between red and blue (e.g., magenta, pink), a color between red, blue and green (i.e., White), or the like is demanded, rather than a source of monochromatic light as described above. As has been described above, typical LEDs can only achieve a source of monochromatic light, and thus for their source of light a fluorescent lump, an incandescent lamp or the like is used and it has thus suffered short lifetime, large sizes and low luminosity.
In contrast, the present inventors have succeeded in obtaining white light, pink light, magenta light and the like from a single LED using a ZnSe-based homoepitaxial layer formed on a ZnSe substrate, as has been described in Japanese Patent Laying-Open Nos. 2000-82845 and 2000-150960.
A ZnSe substrate which is doped with iodine, aluminum, chlorine, bromine, gallium, indium or the like can provide conductance of n-type, and when it is illuminated with light of shorter than 510 nm, ZnSe can provide a so-called self-activated (SA) luminescence, a broad light emission having a center of an emission wavelength in a range of 550 nm to 650 nm. This light emission appears to be yellow, Mars yellow, or the like.
The central wavelength and the half-width of emission spectrum of SA luminescence can be adjusted by the species, amount or the like of the dopant introduced. Furthermore, a light emission structure of ZnSe used as a matrix can be formed on a ZnSe substrate through homoepitaxy. In this light emission structure, ZnSe or ZnCdSe or ZnSeTe can be used for an active layer to form a high bright LED emitting light at a wavelength from 460 nm to 510 nm for blue color or blue-green color.
FIG. 1 shows a concept of the present LED. In the figure an epitaxial light emission structure provides blue or blue-green light emission, of which that emitted toward the substrate is absorbed by a ZnSe substrate 1 and excites optically SA light emission and thus provides yellow, Mars yellow or red light. The former light emission and the latter light emission can be combined together to obtain emission of light of white, pink, magenta or other similar neutral colors.
In the present LED, however, the emitting point and the spatial distribution of the blue or blue-green emission at the epitaxial light emission structure are different from those of the yellow or Mars yellow light emission (that is, fluorescence) at the ZnSe substrate. Thus depending on the direction of the light radiating from the LED the former light emission and the latter light emission are mixed at different ratios and the resultant light emission varies in color disadvantageously.
So a particular improvement is required to also adjust the ratio in intensity between the former light and the latter light to adjust the color of the light emitted from the LED. These issues are not limited to ZnSe-based LEDs for white color and they are common to ZnSe-based LEDs for pink, magenta and other similar neutral colors, and any multi-color LEDs having a substrate of other materials than ZnSe and which are emitting light by fluorescence.
These issues can be addressed, as described in Japanese Patent Laying-open No. 2000-82845 for example by fixing the epitaxial light emission structure down to a stem (epi-side down configuration) which is a frame of the device, to allow light to be radiated through the substrate. Another example comprises a ZnSe substrate 1 which entirely encloses the device.
These approaches, however, contribute to increased cost and disadvantageously prevent typical LED production lines from being used as they are.
The present LED is also be expected to be applied to displaying in mobile gear, such as a back light of a color liquid crystal display of a cellular phone, an illumination positioned under a key-button thereof, and the like
These applications, however, require that a form of a significantly miniaturized surface-mounted LED be implemented at low cost, which is also a significant problem to be addressed.
The present invention has been made to address the above problems. The present invention contemplates a light emitting device producible by a simple and inexpensive process, and emitting light of white, and magenta, pink and other similar neutral colors having a spatially uniform color distribution and provided in various color tones.
The present light emitting device includes: a base having an electrode; a substrate formed of n-type ZnSe single crystal and mounted on the base; an epitaxial light emission structure comprised of a compound crystal relating to ZnSe and serving as a matrix of said light emission structure. The epitaxial light emission structure is secured on the substrate and emits light in response to an electric current passing through the light emission structure. A reflector is provided for correcting the difference in between the spatial distribution of the light emission intensity of the light emitting from the epitaxial structure and the fluorescence light emitting from the substrate.
The reflector scatters the fluorescence from the substrate in a direction of light emitted by the epitaxial light emitting structure. Furthermore the reflector is provided on the base located around the substrate.
Preferably the reflector has an optical reflectance varying to adjust a ratio in intensity between the light emission from the epitaxial light emission structure and the fluorescence from the substrate. Two colors of light can thus be mixed at a varying ratio to obtain a mixture of colors in a desired color tone.
The reflector includes resin of white color, resin having a light scattering medium buried therein, and resin having a surface covered with film of metal, and the base includes a printed circuit board formed of resin and having a patterned interconnection.
The present light emitting device emits light of white color ranging from yellowish white color through bluish white color, or a neutral color including magenta, pink and the like.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.